1. Field of the Invention
The present invention relates to differential signaling systems, and more specifically to correcting for signal mismatch.
2. Background of the Related Art
Components of computer systems communicate along signal paths, such as along electrical traces etched on a circuit board substrate and conductors in electrical connectors. Electronic signals are typically generated as an analog or digital waveform comprising a fluctuating voltage. Signals can be transmitted using either a single-ended system or a differential system. A single-ended system transmits a signal along one signal path, using a fluctuating voltage that a receiver compares with a fixed reference voltage. A differential signaling system, by comparison, transmits two signals sent on two complementary signal paths. The receiver reads the difference between the two complementary signals, ignoring their voltages with respect to ground.
Single-ended systems and differential systems each have inherent advantages and disadvantages. For example, single-ended signaling systems use higher voltages than differential systems. The comparatively large voltages help filter noise because routine electromagnetic interference is unlikely to induce voltages large enough to be misinterpreted as a signal. However, capacitance and inductance effects filter out high-frequency signals that limit the speed of single-ended systems. By comparison, differential systems can use lower voltages because voltages with respect to ground are ignored. Accordingly, minor changes in ground potential between the driver and receiver do not affect the receiver's ability to detect the signal. Ignoring changes in ground potential gives a differential signaling system twice the noise immunity of a single-ended system. Differential systems also consume less energy as a result of the lower voltages used.
Differential signaling systems can carry higher-frequency (also known as higher-speed) signals. However, higher signal speeds are accompanied by the potential for increased signal mismatch between the complementary signal paths. The higher speeds of modern electronics have led to increasingly stringent length-matching requirements in differential signaling systems. Some older circuit board designs that remain in use, sometimes referred to as “legacy” boards, may not comply with the length matching requirements of newer, higher-speed electronic systems. As a result, some legacy boards are not electrically compatible with newer electronic components, despite being physically plug-compatible. For example, a conventional midplane or backplane, which provides communication paths between components of a server chassis, such as between servers and support modules, may not be capable of handling the upper range of signals speeds that some newer servers are capable of generating.